Medium tension circuit-breakers are known having a thermal expansion volume ("thermal volume" for short) in which gas, on being heated by the arc which develops when the arcing contacts separate, expands and blasts the arc on current zero.
It is known that implementing such apparatus gives rise to the following difficulties:
when interrupting low currents (e.g. currents not greater than the nominal current in the line or the installation in which the circuit-breaker is inserted), the pressure build-up may be insufficient or, on the contrary, too great, depending on the size of the thermal volume. If the thermal volume is large, then the pressure build-up is small and the blast may be insufficient. If the thermal volume is small, then the pressure build-up is large, but the blast duration may be insufficient to give good efficiency; and
when interrupting high currents (e.g. short-circuit currents), the pressure build-up and the temperature rise in the gas must not be too large, since this could lead to interruption failure.
In order to solve this problem, proposals have been made, in particular in the document EP-A-0315505, to provide an interrupting chamber (used as a thermal volume) which is variable in volume, depending on the strength of the current to be interrupted.
This is achieved by replacing the fixed arcing contact usually found in circuit-breakers, by a semi-moving contact connected to a piston which is pushed back by an opposing spring.
Depending on the strength of the current to be interrupted, the piston is displaced to a greater or lesser extent, giving rise to a larger or smaller thermal volume.
Such apparatus suffers from a drawback.
When interrupting high currents, the full and rapid return of the semi-moving contact that the spring is incapable of limiting, produces an exaggerated lengthening of the arc and this causes the temperature of the gas to rise excessively, entailing a risk of interruption failure and of major pollution of the insulating gas which could lead to the failure of subsequent interruptions.
An object of the present invention is to provide a medium tension circuit-breaker which does not suffer from this drawback.
Another object of the invention is to provide a circuit-breaker in which, for interrupting high currents, double blasting is performed, i.e. one blast on each of the roots of the arc.
Another object of the invention is to provide a circuit-breaker in which the thermal expansion of the gas is limited, so as to reduce the risks of interruption failure.